Making circular actions without inserting keyframes manually?
Method for a primarily 2D context.


 RMB select the foot bone and go into edit mode of the armature. &#8633; Tab
Create a parent bone at the position, you wish to rotate the foot around. &#8679; ShiftA Bone.   RMB select the foot, then &#8679; Shift RMB select the parent, and parent them with an offset. &#9096; CtrlP > Keep Offset.

Select the child and in the Properties context, in the bone tab, in the Relations section, uncheck Inherit Rotation.

Go into Pose Mode &#9096; Ctrl&#8633; Tab. Select the parent, key the parent at a rotation of 0°, then at a rotation of 360° a few frame later. Locate the keys in the dopesheet and select them. Press &#8679; ShiftE > Make Cyclic, to make the rotation action loop. Press T > linear, to transform it into a linear motion.

Alternatively to the cyclic repetition, you could press &#8679; ShiftE > Linear Extrapolation.